1. Field of the Invention
This invention relates to fluid vaporization and in particular to vaporization by means of shell and tube vaporizers.
2. Description of the Prior Art
To provide efficient storage of large volumes of fluids, such as oxygen, nitrogen, methane, ethane, propane, natural gas, etc., the fluids are conventionally stored under atmospheric pressure in low temperature, or cryogenic, liquid form. To utilize such fluids in gaseous form, it is necessary to vaporize, or evaporate, the liquid by pumping the liquid from the storage tank to a suitable vaporizer. Such vaporizing raises a number of problems due to the fact that the liquid is originally sub-cooled whereas the vaporized fluid comprises a superheated gas.
One problem arising from such vaporization has been the difficulty of maintaining even parallel channel flow distribution through the vaporizer heat exchanger tubes. Failure to maintain such an even flow distribution results in reduced capacity of the vaporizer. Further, such failure may cause mechanical problems resulting from the uneven thermal expansion of different portions of the vaporizer.
One conventional use of such vaporizers is in the vaporization of LNG (liquid natural gas).
Conventionally in such vaporizers, the heat exchange fluid is brought to the operating temperature within the vaporizer prior to the introduction of the liquid to be vaporized. The liquid to be vaporized is then pumped from a storage tank wherein it is maintained at a saturated liquid state, thereby boosting the liquid up to system pressure and resulting in a higher saturated temperature. As there is substantially no heat input into the liquid before it is introduced into the vaporizer, the fluid is introduced as a subcooled liquid.
While a small amount of vaporization may occur in the delivery pipes, the subcooled liquid is primarily vaporized when it contacts the heat exchanger tube surfaces within the vaporizer. As these heat transfer surfaces are relatively hot, rapid boiling takes place which causes a mixing of the liquid sufficiently to raise substantially the entire body of liquid to the saturation point. Vigorous evaporation then takes place so as to produce a substantially 100% gaseous fluid output. Superheating of the gas may take place by heat transfer to the gas prior to discharge from the vaporizer.
Nonuniformity of the rate of flow of the fluid through the different heat exchanger tubes of the vaporizer may result in such a system because of the nonlinearity of the pressure drop thereacross which may have a negative slope at some mixture combination. The pressure drop relationship is illustrated in the following curve: ##EQU1##
As shown in the pressure drop curve, when the subcooled liquid is introduced into the parallel heat exchange tubes, it is possible to have three different mass flow rates with equal pressure drops notwithstanding the fact that the heat exchanger tubes have similar geometry and heat transfer characteristics. More specifically, in the case of a low mass flow rate, a greater amount of vapor is formed, resulting in a decreased mass density. This condition, at a given pressure drop, corresponds to point 3 on the above curve. In the case of a high mass flow rate, less vapor is formed, thereby resulting in an increased mass density illustrated at point 1 on the curve. At an intermediate condition of mass flow, an intermediate mass density results, as shown at point 2 on the curve. As can be seen from the curve, the vapor loads between points 1 and 3 are quite sensitive and somewhat unstable.
In the above described type of vaporizer, the resistance to flow, or pressure drop, across the vaporizer controls the distribution of flow through the individual heat exchanger tubes. As indicated above, at least three separate mass flow rates may exist for a given pressure differential. Thus, the delivery of gas from the vaporizer may be nonuniform with certain portions of the gas having a higher temperature than others.